Andrew, the thickness can be varried to cord length. 3" on a full symetrical airfoil on a 3D plane with a 80 wing span isn't far out of the ball park. My Ruperts Dad has a 60" wing span and the airfoil is 2.5" thick. The plane fits your requirements. My cord is 17" at the root and 15" at the tips. The main wing is a rectangle with tapered ailerons that are about 25% of the cord at the root. The thickness is 2.5" so your 3" for you longer wing sounds OK to me.

Don